1. Field of the Invention
This invention relates to battery voltage monitoring apparatus for smoke detectors and the like and, more particularly, to battery voltage monitoring means for producing both a permanent warning signal and periodic warning signals in response to the detection of a low battery voltage for a period of time.
2. Description of the Prior Art
In modern smoke detectors of the ionization type, low voltage batteries are often used as the power source for the smoke detection apparatus and the associated alarm apparatus for sounding a warning horn or otherwise communicating a warning signal when the smoke level exceeds a predetermined level. While these detectors are typically designed to draw extremely small currents during normal operation, the batteries are eventually depleted of a significant portion of their stored energy. If permitted to remain as the power source, such a battery would be incapable of providing for an extended period the energy required to produce the warning signals in the event of a fire. Eventually, the battery would go completely dead and even be incapable of supporting the sensing function of the smoke detector. It is therefore imperative that the battery be replaced while it still has sufficient stored energy to support the required sensing and warning function of the smoke detector. With this in mind, efforts have been made in the past to detect when the battery is depleted of a certain amount of its energy and produce in response thereto appropriate warning signals. These efforts have been enhanced by the appearance on the market of batteries in which the output voltage permanently drops a significant and detectable amount after the depletion of a predetermined amount of the initial battery energy. For example, Mallory Battery Model No. 304116 has an initial no load voltage of approximately 12.3 volts, which drops somewhat abruptly and permanently to approximately 10.6 volts after 60 percent of its energy has been expended.
A typical warning approach is to provide means for sensing when the battery energy has declined to a predetermined level and producing in response thereto periodic warning signals, such as the sounding of a warning horn of the smoke detector once a minute or at other frequent periods for an extended period of 7 or more days. If the battery voltage detection and warning system is to be powered by the same battery used for the smoke detection and warning system, it must also place a low current drain on the battery in order for the already depleted battery to support the periodic warning signals for an extended period without depleting the battery energy to such an extent that it is incapable of supporting the smoke detection and warning functions of the smoke detector. In spite of the extended period during which a low battery warning signal is produced in such systems, these approaches are not entirely effective in warning the homeowner or other user of the low battery energy condition since the warning signals cease when the battery is fully depleted. If the homeowner is absent throughout the warning period, he will not be warned upon his return that the battery is fully depleted and that the smoke detector is therefore inoperative.